btrfs-progs: fix improper error handling in btrfs filesystem usage

I was seeing test-cli/016 failures because it claimed we were getting
EPERM from the TREE_SEARCH ioctl to get the chunk info out of the file
system.  This turned out to be because errno was already set going into
this function, the ioctl itself wasn't actually failing.  Fix this by
checking for a return value from the ioctl first, and then returning
-EPERM if appropriate.  This fixed the failures in my setup.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

cmds/filesystem-usage.c

@@ -145,7 +145,7 @@ static int load_chunk_info(int fd, struct chunk_info **chunkinfo_ret,
 	struct btrfs_ioctl_search_key *sk = &args.key;
 	struct btrfs_ioctl_search_header *sh;
 	unsigned long off = 0;
-	int i, e;
+	int i;
 
 	memset(&args, 0, sizeof(args));
 
@@ -168,11 +168,9 @@ static int load_chunk_info(int fd, struct chunk_info **chunkinfo_ret,
 
 	while (1) {
 		ret = ioctl(fd, BTRFS_IOC_TREE_SEARCH, &args);
-		e = errno;
-		if (e == EPERM)
-			return -e;
-
 		if (ret < 0) {
+			if (errno == EPERM)
+				return -errno;
 			error("cannot look up chunk tree info: %m");
 			return 1;
 		}